The present invention relates to an advance control system for use in a drilling apparatus such as rock drilling apparatus.
Generally, a rock drilling apparatus includes, as shown in FIG. 1, a drill 2 slidably mounted on an elongated casing 4 which is generally called a cell. A driving rod 6 extending outwardly from the drill 2 is supported on a support 8 rigidly mounted at one end of the cell 4. The end of the rod 6 remote from the drill 2 is provided with a bit 10 which substantially drills a hole in the rock by the rotation thereof while striking impacts are applied axially to the rod 6. Mechanism for effecting the rotation and applying the impact to the bit 10 through the rod 6 is omitted. The drill 2 has a lower end portion extending into the cell 4 and engaged to a threaded shaft 12 which extends longitudinally and rotatably in the cell 2. The left-hand end of the threaded shaft 12 remote from the end provided with the support 8 is connected to a feed motor M operated by a fluid pressure such as air. When the feed motor M is driven to rotate the shaft 12 in one direction, the drill 2 is forcibly moved in one direction such as the forward direction whereas, when the feed motor M is driven to rotate the shaft 12 in the other direction, the drill 2 is forcibly moved in the other direction such as the reverse direction. It is to be noted that the cell 4 is preferably mounted on a vehicle (not shown) through a suitable boom (not shown).
For carrying out a drilling operation at a constant speed with a constant torque, the advancing force of the drill 2 effected by the motor M should be adjusted to cope with the hardness of the rock. For example, when drilling a hole in a very hard rock or in a clay, the drill 2 is applied with a high advancing force. On the other hand, when drilling a hole in a sandy or soft rock, the drill 2 is applied with a low advancing force. However, in the case of hard rock, if the advancing force exceeds the optimum force, the rotating torque on the rod 6 becomes very high and, at the same time, the rotational speed of the rod 6 becomes small or even equal to zero due to the firm contact between the rock and the bit 10. On the other hand, in the case of soft rock, there will hardly be any drilling resistance from the soft rock and the rod 6 rotates at a very high speed. Therefore, the tip of the bit 10 will be easily worn-out by the abrasion against the rock. In addition, other components of the drill 2 will also be worn-out by the high speed rotation of the rod 6. Therefore, it is preferable to precisely control the operation of the drilling apparatus. Generally, in the drilling apparatus, when the rod is rotated at a very high speed, the rotational speed of the rod has to decrease with the increase of the drill advancing force. On the other hand, when the rod is rotating at a very low speed, the rotational speed of the rod has to increase with the decrease of the drill advancing force. Therefore, in order to stabilize the rotational speed of the rod, it is necessary (i) to detect the rotational speed of the rod; (ii) to increase the advancing force in the case in the increase of the rotational speed; and (iii) to decrease the advancing force in the case of the decrease in the rotational speed. Since the advancing force is in relation to the air pressure supplied to the feed motor M, the advancing force can be controlled by the air pressure supplied to the feed motor M.
From this point of view, there have been proposed various advance control systems for use in a drilling apparatus. One conventional control system detects the exhaust or suction pressure at a rotation sleeve which rotates together with the rod for the continuous detection of the rotational speed of the rod. However, in this control system, there have been such disadvantages that the rotational speed can not be detected with high accuracy and that a nozzle used for detecting the pressure is often choked by dust. Another conventional type of advance control system for use in a drilling apparatus is diagramatically shown in FIG. 2.
In FIG. 2, the conventional control system which is disclosed in the Japanese Laid-Open Utility Model Publication No. 50-121001 published on Oct. 3, 1975 includes a rotary disc 14 coaxially connected to the rod 6 (FIG. 1) and a proximity or contactless switch 16 of any known type positioned closely adjacent the peripheral edge of the disc 14. The disc 14 has a plurality of recesses 14a, (for example, eight as shown in FIG. 2), formed around the peripheral edge portion of the disc 14 at a predetermined pitch. The proximity switch 16 detects the recess 14a and produces a low signal when the recess 14a faces the proximity switch 16. Therefore, the proximity switch 16 produces a train of pulse signals as a result of the rotation of the disc 14. It is understood that the frequency of the pulse signal is in proportion to the speed of rotation of the rod 6. The control system further includes an oscillator 18 which produces a pulse signal having a very long pulse duration. The oscillator 18 is coupled to a synchronizing pulse producing circuit 20 which produces a single shot pulse through a line P1 simultaneously with the leading edge and trailing edge of the pulse from the oscillator 18 and also produces a single shot pulse through a line P2 immediately after the pulse has been produced on the line P1. The synchronizing pulse producing circuit 20 further produces a single shot pulse through a line P3 immediately after the pulse has been produced on the line P2. A counter 22 is provided for counting the number of pulses produced from the proximity switch 16 in a predetermined period of time controlled by the synchronizing pulse producing circuit 20. The counter 22 starts counting the train of pulses from the proximity switch 16 upon receipt of one shot pulse from the synchronizing pulse producing circuit 20 through the line P3. The counting of the pulse in the counter 22 is effected until the counter 22 receives the one shot signal from the synchronizing pulse producing circuit 20 through the line P1. Immediately thereafter, upon receipt of the one shot pulse from the line P2, the counter shifts the counted number of the pulses to a comparator 24. The one shot pulse from the line P2 is also applied to a register 26 for shifting a prearranged number stored in the register 26 to the comparator 24. This prearranged number corresponds to the desired number of pulses to be produced from the proximary switch 16 during the predetermined period of time. In the comparator 24, the number from the counter 22 is compared with the number from the register 26 and the difference therebetween is fed to a control circuit 28 and further to an air pressure control circuit 30. In the case where the number from the counter 22 exceeds the prearranged number from the register 26, it is understood that the bit 10 of the drill is rotating at a speed higher than the required speed. In this case, the control circuit 28 causes the air pressure circuit 30 to provide more pressure to the motor M so that the drill can advance with a greater force. On the other hand, in the case where the number from the counter 22 falls below the prearranged number, it is understood that the bit 10 of the drill is rotating at a speed lower than the required speed. In this case, the control circuit 28 causes the air pressure circuit 30 to provide less pressure to the motor M so that the drill can advance with a less force. In the case where the number from the counter 22 is equal to the prearranged number, the control circuit 28 is so actuated as to maintain the drilling speed at the required speed.
According to the above described system for controlling the advance of the drilling apparatus, the detection of the number of rotation of the disc 14 can be carried out with higher accuracy with the increase of the number of the recesses 14a formed in the disc 14. In view of this, the conventional controlling system has 8 to 10 recesses formed in the disc 14. However, because of the vibration and impact produced during the drilling operation, a false train of pulses are often produced from the proximity switch 16. The description is now directed to such a false signal.
During the drilling operation, when the bit 10 is rigidly caught by the rock, the bit 10 would be held tightly inside the rock whereas the rod 6 continuously receives impacts and rotating force. In this case, the rod 6, particularly the portion accommodated inside the drill 2 is apt to be vibrated by such rotating force. Accordingly, the disc 14 rigidly connected to the rod 6 is also vibrated. Therefore, the recess facing the proximity switch 16 alternately comes close to and moves away from such switch 16 to produce a false train of pulses from the switch 16. The frequency of such false pulses is in relation to the frequency of the vibration. Similar false pulses are produced by the impact applied to the rod 6.
According to the tests carried out by the present inventor, three different models of rock drilling apparatus TY-1, TY-2 and TY-3 have been examined to find for each model (i) the proper rotational speed of the drill; (ii) the number of false pulses produced per minute by the vibration; and (iii) the number of false pulses produced per minute by the impact. The inner diameter of the impact cylinder used for models TY-1, TY-2 and TY-3 are 120 mm, 110 mm and 90 mm, respectively. The drilling operation is effected on granite with the operative pressure ranged between 3 kg/cm.sup.2 to 7 kg/cm.sup.2. The results are shown in FIGS. 3a, 3b and 3c.
In FIG. 3a, a region A1 covering 80 to 220 rpm indicates a range of rotation of the rod 6 in which the drilling operation is carried out by the use of model TY-1. Five circle dots A2, A3, A4, A5 and A6 indicate the number of rotation effected by the operative pressure of 3, 4, 5, 6 and 7 kg/cm.sup.2, respectively, during no-load operation, that is, when the bit 10 is free from any object. Similarly, for model TY-2, the drilling operation mode takes places in a region B1 covering 50 to 230 rpm while no-load operation mode for the operative pressure of 3, 4, 5, 6 and 7 kg/cm.sup.2 are shown by dots B2, B3, B4, B5 and B6, respectively. Likewise, for model TY-3, the drilling operation mode takes place in a region C1 covering 30 to 140 rpm while no-load operation mode for the operative pressure of 3, 4, 5, 6 and 7 kg/cm.sup.2 are shown by dots C2, C3, C4, C5 and C6, respectively. Therefore, the available rotational speeds for models TY-1, TY-2 and TY-3 range from 80 to 400 rpm, from 50 to 300 rpm, and from 30 to 260 rpm, respectively. Therefore, in the case where the disc 14 has eight recesses formed therein, the pulse repetition frequency (PRF) from the proximary switch 16 for models TY-1, TY-2 and TY-3 are in a range which is 8 times the number given above, that is, 640 to 3,200 ppm, 400 to 2,400 ppm and 240 to 2,080 ppm, respectively.
In FIG. 3b, regions A7 and A8 covering from 850 to 3,100 ppm indicate the range of the number of false pulses produced per minute from model TY-1 during the vibration thereof. Similarly, regions B7 and B8 covering from 1,100 to 2,000 ppm indicate the range of the number of false pulses produced per minute from model TY-2 during the vibration thereof. Likewise, regions C7 and C8 covering 700 to 2,150 ppm indicate the range of the number of false pulses produced per minute from model TY-3 during the vibration thereof.
It is to be noted that the vibrations in the regions A7, B7 and C7 take place when the bit 10 is completely stuck in the rock to allow no movement of the bit 10 while the vibration in regions A8, B8 and C8 take place when the bit 10 is stuck in the rock, but can make a slight movement.
In FIG. 3c, a region A9 covering 2,100 to 2,600 ppm indicates a range of the number of false pulses produced per minute from model TY-1 by the impact. Similarly, a region B9 covers 1,800 to 2,400 ppm and a region C9 covers 1,500 to 1,900 ppm for the false pulses produced by the impact in models TY-2 and TY-3, respectively.
As apparent from the result, the problem with the conventional control system described with reference to FIG. 2 is that the train of pulses properly produced as a result of rotation of the disc 14 cannot be distinguished from the train of pulses falsely produced as a result of vibration or impact, since the frequency of both trains of pulses fall approximately in the same region.
Although it is simply suspected that the reduction in number of recesses in the disc 14 accordingly reduces the frequency region of the train of pulses produced thereby, such reduction in number of the recesses lowers the accuracy in the detection of the rotational speed.